The invention relates to programming and reading a nonvolatile memory cell comprising a dielectric and/or diode and a phase-change element in series.
Phase-change materials such as chalcogenides have been used in nonvolatile memories. Such materials can exist in one of two or more stable states, usually high-resistance and low-resistance states. In chalcogenides, the high-resistance state corresponds to an amorphous state, while the low-resistance state corresponds to a more ordered crystalline state. The conversion between states is generally achieved thermally.
Integrated circuit memories are typically large arrays of memory cells connected between bit lines and word lines. In order to achieve reliable programming and reading of the memory cells within the array, memory cells selected to be programmed or read must be isolated from memory cells that are not selected. It sometimes occurs that cells adjacent to selected cells are disturbed during a write operation, or that cells on the same word line or bit line as selected cells may be disturbed during the write operation. This problem becomes increasingly important as operating voltages are decreased, write speeds are increased, memory cell densities are increased, and array sizes are increased.
There is a need for improved methods of programming and reprogramming the memory cells. The cells must be programmed quickly, using low current, and in a manner that assures proper writing and reading of memory cells in the array. One problem when writing and reading memories is that switching occurs very fast, and sometimes a word line or bit line that was recently selected may not have returned to its resting voltage when the next word line or bit line is selected, and selecting a new word line or bit line may cause unintentional programming (or reading) of a cell connected to a previously selected word line or bit line.